Geoscience Reference
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quantified, in the Irish Sea as a whole, and beginning long before I
arrived on the Welsh coast, the ecosystem does appear to have been
turning to jelly. A research paper links this change to a combination
of warming waters and overfishing, in particular the herring fishery
off the coast of Ireland in the 1970s. 36 There, fishermen using paired
trawlers pursued juvenile herring to turn into fishmeal:37 37 they were
ground into feed for pigs and chickens or fertilizer for crops and
lawns. I struggle to find the words required to describe the wasteful-
ness of this operation.
This, the study suggests, might have helped to create 'a cascading
regime shift', which tipped the balance in favour of jellyfish. With
fewer competitors for the plankton they eat, they were able to prolif-
erate. As the herring population begins to recover, this might go into
reverse, though if the mackerel have gone, the jellyfish could once
more have been released from competition.
Similar shifts have taken place, for the same reason, off the coasts
of Namibia and Japan and in the Black, Caspian and Bering seas. 38 In
all these cases, small plankton-eating fish, such as herring, sardines
and anchovies, which both competed with the jellyfish for prey and,
perhaps, ate the young jellies, have been greatly reduced by fishing,
and animate gloop has swarmed into the breach. Jellyfish can also
survive much better than fish in water whose oxygen has been depleted
by plankton blooms: they are among the few lifeforms that can live in
the dead zones now developing in many seas. They also have a pecu-
liar ability to resist the destruction caused by fishing nets: they can
regenerate themselves after they have been shredded.
One paper warns of a 'never-ending jellyfish joyride'. 39 Beyond a
certain density, jellyfish inflict on depleted populations of herrings
and similar species what the herrings inflict on depleted cod: they pre-
vent them from recovering by eating their eggs and young. This allows
the jellyfish to proliferate further, wiping out other fish and threaten-
ing to replace them with a jelly monoculture.
The lesson emerging repeatedly from studies of the ecosystems of
land and sea is that plagues take place when keystone species are
removed. When they have not been heavily exploited, natural systems
can, it seems, prevent explosions of native species and control inva-
sions of most exotic species. They are also better able to withstand
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